Mice-bearing methylcholanthrene-induced fibrosarcomas have lymphoid cells which suppress immune responses to cells of the same tumor in vitro. Thymus-derived lymphocytes from these mice synthesize a factor which blocks cytolysis of homologous tumor cells by specifically immune lympohid cells. This project proposes to study the mechanism of this suppression and to determine if inactivation of the suppressor cells in vivo will augment rejection of transplanted tumors. Hybridization of T cells from tumor-bearing mice with BW5147 cells will be used to obtain monoclonal suppressor factor specific for antigens of the immunizing sarcoma. The monoclonal suppressor factors will then be used to determine the mechanism of the suppression in 51Cr-release assays in vitro and in vivo assays measuring tumor rejection. The cells required for the expression of this suppression will be identified. Antibodies will be raised to the suppressor factors. Future studies will determine if these antisera inactivate T suppressor cells in vitro and if treatment of tumor-bearing mice with these antisera removes T suppressor cells specific for the tumor and leads to retarded tumor growth.